1. Field of the Invention
This invention relates to an automatic exposure control device of a camera, and more particularly to an automatic exposure control device of a camera having a metering device for dividing a phototaking picture plane into a plurality of areas, processing the metering output for each area and determining the exposure of the entire picture plane.
2. Description of the Prior Art
A metering device for dividing the phototaking picture plane into a plurality of areas, processing the metering output for each of the areas and operating a metering output for determining the exposure of the entire picture plane (hereinafter referred to as the multimetering device) is already known from Japanese Laid-open Patent Applications Nos. 12828/1977 and 52419/1978. As techniques similar to these, the assignee of this invention filed Japanese Patent Applications Nos. 23019/1979, 23020/1979 and 23021/1979. This multimetering device has an advantage that in photographing an ordinary object having a relatively small brightness difference between parts thereof as well as a special object having a great brightness difference between parts thereof, such as, for example, a figure under counter-light or a figure under spot light illumination, such object is automatically discriminated and an exposure suitable therefor is obtained.
The techniques disclosed in the aforementioned Japanese Laid-open Patent Applications Nos. 12828/1972 and 52419/1978 are as follows. The maximum value P.sub.max and minimum value P.sub.min of a plurality of bits of brightness information (metering outputs) obtained from the metering areas of the phototaking picture plane are detected, whereby the difference P.sub.max -P.sub.min and the mean value P.sub.mean =P.sub.max +P.sub.min /2 are operated. This difference P.sub.max -P.sub.min is compared with a reference value .delta. by a comparing circuit and judgment is made that when P.sub.max -P.sub.min .ltoreq..delta., the object is one having not so great a brightness difference between parts thereof and that when P.sub.max -P.sub.min &gt;.delta., the object is one having a great brightness difference between parts thereof. When P.sub.max -P.sub.min .ltoreq..delta., the mean value P.sub.mean of the plurality of metering outputs is put out, and when P.sub.max -P.sub.min &gt;.delta., discrimination is made as to whether the object is a white back (an object having a great deal of light portion in the background like a figure under counter-light) or a black back (an object having a great deal of dark portion in the background like a figure under spot light illumination) and then P.sub.max or P.sub.min is put out. The P.sub.mean, P.sub.max or P.sub.min provides the final proper exposure value which determines the exposure of the entire picture plane.
The discrimination between a white back and a black back is accomplished by comparing the mean value P.sub.mean with the median value (P.sub.max +P.sub.min)/2. That is, when the median value is greater than the mean value, the object is often one the whole of which is dark and therefore such object is decided as a black back and in the converse case, the object is decided as a white back. In the case of a black back, namely, (P.sub.max +P.sub.min)/2.gtoreq.P.sub.mean, the maximum value P.sub.max is put out and in the case of a white back, namely, (P.sub.max +P.sub.min)/2&lt;P.sub.mean, the minimum value P.sub.min is put out.
On the other hand, an aperture control device of the type in which while the aperture of the lens is being stopped down from the open aperture diameter toward the minimum aperture diameter, the light from an object passed through the aperture opening is measured by a photoelectric element and when the brightness of the object and the aperture diameter being stopped down assume a relation corresponding to a preset shutter speed, the stop-down operation is blocked to automatically adjust the aperture, whereby automatically controlling the exposure is already known from Japanese Patent Publication No. 23779/1974 (corresponding U.S. Pat. No. 3,829,867 and German Pat. No. 2,133,212).
Thus, it occurs to mind to achieve further improved automatic exposure control by combining the multimetering device and this automatic exposure control device. In this case, however, attention must be paid to the following. That is, in some cases, the operated metering output of the multimetering device (multimetering output) is not coincident with any of the metering outputs in the metering areas before the operation, depending on the conditions of the object. In contrast, where said combination is achieved as by replacing the photoelectric element of the automatic exposure control device with part or whole of the photoelectric element of the multimetering device (the replaced photoelectric element of the multimetering device will hereinafter be referred to as the monitoring photoelectric element), the stop-down blocking period must be determined by effecting the stop-down metering with the aid of the monitoring photoelectric element and therefore, if the multimetering output and the metering output of the monitoring photoelectric element are not coincident with each other before the stop-down, a control error corresponding to that output difference occurs to the controlled aperture opening. For example, where the monitoring photoelectric element is designed to meter the central area of the picture plane, when the multimetering device generates a multimetering output to adjust the exposure with a major object lying at a corner of the picture plane, the automatic exposure control device blocks the stop-down when the relation between the aperture obtained from the output of the monitoring photoelectric element and the brightness of the object in the center of the picture plane has come to correspond to a preset shutter speed and therefore, the aperture control with respect to the major object lying at a corner of the picture plane is not effected.
What has been described above also applies to a case where the multimetering photoelectric element does not act also as the monitoring photoelectric element, for example, a case where the monitoring photoelectric element meters the reflected light from a film surface or a shutter curtain surface and the multimetering photoelectric element meters the light passed through the focus plate of the camera.
Moreover, in Japanese Laid-Open Patent Application No. 12828/1977, it is further disclosed that the main switch SW is interlocked with the shutter releasing operation and when this main switch is closed, a channel change-over circuit CS is brought into operation so as to close progressively and according to the time division analog switches AG1, AG2, . . . AGn at the front stage. Therefore, the logarithmic voltages P1, P2, . . . Pn which are fed from the terminals of the analog switches are introduced into a circuit PH for detecting and holding the maximum output value and a circuit VH for detecting and holding the minimum output value, so that the maximum voltage P max and the minimum voltage P min may be detected and held by these circuits PH and VH.
When all of the analog switches AG1, AG2, . . . AGn are closed through the channel change-over circuit CS, the outputs P1, P2, . . . Pn fed from the other terminals of the analog switches are introduced into a circuit MH for operating and holding the average value so that the average value P mean of the outputs P1, P2, . . . Pn may be operated and held by the circuit MH.
When the above-mentioned process is finished, the subtraction between the minimum and the maximum values (P max-P min) is accomplished by an operational amplifier OP1, while the median between the maximum and the minimum values (P max+P min/2) is obtained by another operational amplifier OP2. Then, the difference voltage resulted from the subtraction between the minimum and the maximum values (P max-P min) is compared with a predetermined voltage .delta. by a comparator CP1 arranged at the subsequent stage, and it can be judged that P max-P min.gtoreq..delta., the object has not so large a difference in its luminance on each surface section thereof and that when P max-P min&gt;.delta., the object has a large difference in its luminance on each surface section thereof.
For example, in the case of P max-P min.ltoreq..delta., the comparator CP1 is kept in its OFF stage and an analog switch AS3 at the back stage is closed by an inverter G3 so that the average value operation and detecting circuit MH feeds an average value (P1+P2+ . . . +Pn)/n as an output. In the case of P max-P min&gt;.delta., it is determined whether the background is white (the background has many bright portions) or black (the background has many dark portions), combining with the result obtained from the manner which will be described later, so that P max of P min or a value approximating P max or P min is fed, as an output, from the circuit MH.
In the latter case, it is determined whether the background is white or black in such a manner that the median (P max+P min)/2 and the average value P mean are compared with each other by another comparator CP2. Namely, when the median is larger than the average value, it is determined that the background is black since the object has often dark portions overall, while when the average value is larger than the median it is determined that the background is white since the object has often bright portions overall.
In case both of the median and the average value are compared with each other to give a result P max+P min/2.ltoreq.P mean, the other comparator CP2 is put ON. If the comparator CP1 is also in the state of P max-P min&gt;.delta. and is kept ON at this time, an AND gate G1 will be put ON and the analog switch AS1 is closed to thereby feed P max as an output. On the contrary, in case the average value is larger than the median, the other comparator CP2 is put OFF. Since the OFF signal is inverted by an inverter G4 at this time, an AND gate G2 is put ON and an analog switch AS2 is closed to thereby feed P min as an output.
It is also noted that while this publication teaches judging the pattern of an object by comparing the analog quantities in the embodiment shown in the Figures thereof, it also discloses that comparison of digital quantities may be carried out instead.